Many types of earth handling equipment are provided with valve-controlled hydraulic systems for operating implements and drive motors. As an example, the speed and direction of some tractors is controlled by the flow of hydraulic fluid through a hydraulic motor connected to the wheels. Usually a control lever is connected with a valve having two operating positions, such as forward and reverse opposite each other from a center neutral position. As the lever is moved from its neutral position, the valve spool is moved from its neutral position into one of the operating positions. Hydraulic fluid then flows through the valve energizing the motor to move the tractor in the chosen direction.
Unfortunately, many prior control linkages operate the valve spool to quickly energize the motor. With a tractor this can often result in a jerky start or a loss of traction. This is because there is a significant amount of valve spool displacement as the lever is initially moved from its neutral position. A relatively large amount of fluid flows quickly through the valve and the hydraulic motor is then quickly energized, causing the jerky start.
Each unit of lever movement of the lever from its neutral position causes a relatively large amount of initial valve spool displacement from its neutral position. Generally where the lever is rotated x degrees from its neutral position, the valve spool is displaced at a relative rate which is a function of the cosine of x. Thus, the greatest relative rate of valve spool displacement takes place initially during relatively small amounts of lever rotation, i.e. small values of x, when the motor is first being energized. Increasing the total movement of the lever can reduce the initial high rate of valve spool displacement, but results in an unwieldy amount of total lever movement.
As the control lever is first moved, the valve should at first be displaced slowly from its neutral position to provide better control. An initial slow rate of valve spool movement relative to the rate of control lever movement would slowly energize the motor or other valve-controlled device. A faster rate of relative valve spool displacement away from the lever neutral position would allow the valve to be fully opened without excessive total lever movement.
It would be desirable to provide a valve control linkage which avoids the deficiencies of the prior art to provide more effective valve operation. Each unit of initial control lever motion from its neutral position should displace the valve spool relatively less than each unit lever motion in its operating positions. This would provide more controlled initial operation of the hydraulic system. It would also be desirable if the valve control linkage did not require excessive total lever movement. The valve control linkage of the present invention meets these desires.